Method of manufacture of single crystal synthetic diamond material

ABSTRACT

A method of manufacturing synthetic diamond material using a chemical vapour deposition process, and a diamond obtained by such a method are described. The method comprises providing a freestanding synthetic single crystal diamond substrate wafer having a dislocation density of at least 107 cm−2. The synthetic single crystal diamond substrate wafer is located over a substrate holder within a chemical vapour deposition reactor. Process gases are fed into the reactor, the process gases including a gas comprising carbon. Crack-free synthetic diamond material is grown on a surface of the single crystal diamond substrate wafer at a temperature of at least 900° C. to a thickness of at least 0.5 mm and with lateral dimensions of at least 4 mm by 4 mm.

FIELD

The invention relates to the field of manufacturing single crystalsynthetic diamond material, in particular single crystal syntheticdiamond material manufactured using a Chemical Vapour Deposition (CVD)method.

BACKGROUND

Diamond materials may be categorized into three main types: naturaldiamond materials; HPHT (high pressure high temperature) syntheticdiamond materials, and CVD (chemical vapour deposited) synthetic diamondmaterials. These categories reflect the way in which the diamondmaterials are formed. Furthermore, these categories reflect thestructural and functional characteristics of the materials. This isbecause while natural, HPHT synthetic, and CVD synthetic diamondmaterials are all based on a theoretically perfect diamond lattice thedefects in these material are not the same. For example, CVD syntheticdiamond contains many defects unique to the process of CVD, and whilstsome defects are found in other diamond forms, their relativeconcentration and contribution is very different. As such, CVD syntheticdiamond materials are different to both natural and HPHT syntheticdiamond materials.

Diamond materials may also be categorized according to their physicalform. In this regard, diamond materials may be categorized into threemain types: single crystal diamond materials; polycrystalline diamondmaterials; and composite diamond materials. Single crystal diamondmaterials are in the form of individual single crystals of various sizesranging from small “grit” particles used in abrasive applicationsthrough to large single crystals suitable for use in a variety oftechnical applications as well for gemstones in jewellery applications.Polycrystalline diamond materials are in the form of a plurality ofsmall diamond crystals bonded together by diamond-to-diamond bonding toform a polycrystalline body of diamond material such as apolycrystalline diamond wafer. Such polycrystalline diamond materialscan be useful in various applications including thermal managementsubstrates, optical windows, and mechanical applications. Compositediamond materials are generally in the form of a plurality of smalldiamond crystals bonded together by diamond-to-diamond or a non-diamondmatrix to form a body of composite material. Various diamond compositesare known including diamond containing metal matrix composites,particularly cobalt metal matrix composites known as polycrystallinediamond (PCD), and skeleton cemented diamond (ScD) which is a compositecomprising silicon, silicon carbide, and diamond particles.

It should also be appreciated that within each of the aforementionedcategories there is much scope for engineering diamond materials to haveparticular concentrations and distributions of defects in order totailor diamond materials to have particular desirable properties forparticular applications. The present disclosure is concerned with CVDsingle crystal synthetic diamond materials.

CVD processes for synthesis of diamond material are well known. Being inthe region where diamond is metastable compared to graphite, synthesisof diamond under CVD conditions is driven by surface kinetics and notbulk thermodynamics. Diamond synthesis by CVD is normally performedusing a small fraction of carbon (typically <5%), typically in the formof methane although other carbon containing gases may be utilized, in anexcess of molecular hydrogen. If molecular hydrogen is heated totemperatures in excess of 2000 K, there is a significant dissociation toatomic hydrogen. In the presence of a suitable substrate material, CVDsynthetic diamond material can be deposited. Polycrystalline CVD diamondmaterial may be formed on a non-diamond substrate such as a refractorymetal or silicon substrate. Single crystal CVD synthetic diamondmaterial may be formed by homoepitaxial growth on a single crystaldiamond substrate.

Atomic hydrogen present in the process selectively etches offnon-diamond carbon from the substrate such that diamond growth canoccur. Various methods are available for heating carbon containing gasspecies and molecular hydrogen in order to generate the reactive carboncontaining radicals and atomic hydrogen required for CVD syntheticdiamond growth including arc-jet, hot filament, DC arc, oxy-acetyleneflame, and microwave plasma.

A problem with prior art methodologies is how to achieve large areasingle crystal CVD synthetic diamond material. It has been found thatlarge area single crystal diamond can be grown by a process known as“heteroepitaxial growth”. This is where diamond nucleates and growsepitaxially on a non-diamond substrate. Iridium has been found to be asuitable substrate to allow diamond nucleation and growth, but othersubstrates such as silicon, silicon carbide, copper, nickel, rhenium andtitanium carbide have been investigated. U.S. Pat. No. 7,396,408describes such a process. In this case, diamond is grown in a CVDprocess using a silicon carbide or silicon single crystal wafer that hasa layer of iridium deposited on its surface. This is used as a substrateon which to heteroepitaxially deposit and grow diamond. During thegrowth process, diamond crystallites nucleate on the iridium film. Thesecrystallites grow and merge to form a single crystal layer, which iscontinued until a single crystal diamond wafer of the desired thicknessis formed. Typically the dislocation density reduces via dislocationinteractions (fusion and annihilation) as growth proceeds, leading to asingle crystal diamond wafer that has a higher dislocation densityadjacent to the original nucleation face compared to the growth face.

A problem with large area single crystal diamond growth using aheteroepitaxial growth method is that the crystal lattice parametermismatch between the iridium (or other substrate) and the grown diamondcauses a high dislocation density in the resultant diamond wafer which,even after the annihilation and fusion of dislocations in the earlystages of growth, can still be of an order typically observed in naturaltype IIa diamonds (around 10⁷ cm⁻² and greater). Such a high density ofdislocations can be detrimental for certain industrial applications ofCVD diamond, such as low birefringence optical windows, ruling out thedirect use of heteroepitaxially grown CVD diamond for such applications.

A further problem is that the substrate materials used forheteroepitaxial growth can lead to an undesirable incorporation ofimpurities into the grown diamond lattice, such as silicon.

It may further be the case that the growth conditions required to growhigh quality grades of CVD diamond (such as a high microwave powerdensity), are incompatible with the substrate materials used forheteroepitaxial growth. For example, the use of a silicon substrate,with its relatively low thermal conductivity (compared to that ofdiamond), can set a practical limit on the CVD process power density,since higher power densities lead to higher heat fluxes. These can inturn generate thermal gradients within the silicon wafer, which lead toits mechanical failure.

SUMMARY

For certain applications of CVD diamond requiring large area singlecrystals, it is therefore desirable to provide a process for growing ona heteroepitaxially grown single crystal CVD diamond wafer previouslydetached from its non-diamond substrate. Such a process allows theaforementioned problems to be solved: methods for reducing the CVDdiamond dislocation density can be employed (such as those outlined inWO2004/027123); undesirable incorporation of impurities from the use ofnon-diamond substrates such as silicon is avoided; and practicallimitations to CVD process parameters due to the use of non-diamondsubstrates are alleviated.

However, it is known from prior art that CVD diamond growth on singlecrystal substrates with a high density of dislocations, such as thosemanufactured from type IIa natural diamond, or from heteroepitaxiallygrown CVD diamond, can lead to undesirable crack formation, and/or anundesirable transition from single crystal to polycrystalline diamond(see for example, Diamond and Related Materials, Volume 3, Issues 4-6,April 1994, Pages 408-416). Such undesirable effects can occur even forCVD diamond layers grown relatively thinly (around 0.5 mm). It istherefore desirable to provide a process for growing relatively thick,crack-free single crystal CVD diamond on single crystal diamondsubstrates with a high density of dislocations, such asheteroepitaxially grown CVD diamond substrates.

It is an object of the present invention to provide a process forgrowing single crystal CVD diamond with a thickness of at least 0.5 mmthick which is substantially crack-free, on a single crystal diamondsubstrate with a high density of dislocations.

According to a first aspect, there is provided a method of manufacturingsynthetic diamond material using a chemical vapour deposition process.The method comprises providing a freestanding synthetic single crystaldiamond substrate wafer having a dislocation density of at least 10⁷cm⁻². The synthetic single crystal diamond substrate wafer is locatedover a substrate holder within a chemical vapour deposition reactor.Process gases are fed into the reactor, the process gases including agas comprising carbon. Crack-free synthetic diamond material is grown ona surface of the single crystal diamond substrate wafer at a temperatureof at least 900° C. to a thickness of at least 0.5 mm and with lateraldimensions of at least 4 mm by 4 mm.

As an option, the growth conditions are maintained to provide an alphagrowth parameter of no more than 2 or no more than 1.4.

The process gases optionally include hydrogen and not more than 5%oxygen.

The method optionally further comprises the step of growing the singlecrystal diamond substrate wafer heteroepitaxially.

A power density sufficient to achieve a growth rate is optionallyselected from any of at least 4 μm per hour, at least 5 μm per hour, atleast 10 μm per hour and at least 15 μm per hour.

As an option, the method further comprises providing the syntheticsingle crystal diamond substrate wafer by the steps of locating anon-diamond substrate over a substrate holder within a further reactor,the substrate comprising a surface on which the synthetic single crystaldiamond substrate wafer is to be grown heteroepitaxially, feedingprocess gases into the further reactor, and growing the synthetic singlecrystal diamond substrate wafer on the surface of the non-diamondsubstrate.

As a further option, the reactor is the further reactor; in other words,the heteroepitaxially grown diamond substrate wafer may be grown in thesame reactor as the synthetic diamond material, or it may be grown in aseparate reactor.

As a further option, the non-diamond substrate comprises any of iridium,silicon and silicon carbide. Any suitable non-diamond substrate may beused.

As a further option, the method comprises removing a non-diamondsubstrate from the synthetic single crystal diamond substrate waferprior to locating the synthetic single crystal diamond substrate waferover the substrate holder. This minimises any contaminants in the grownsynthetic diamond material that would otherwise arise from thenon-diamond substrate.

As an option, the method further comprises, prior to locating thesynthetic single crystal diamond substrate wafer over the substrateholder within the reactor, processing a surface of the synthetic singlecrystal diamond substrate wafer to reduce surface damage. Optionalexamples of surface processing include polishing, chemical mechanicalpolishing, etching, and laser processing.

The grown synthetic diamond material is optionally oriented atsubstantially {100} relative to the synthetic single crystal diamondsubstrate wafer. However. It will be appreciated that the substrate maybe oriented at any desired angle to present a desired crystallographicplane to the growth surface of the single crystal diamond substratewafer.

The single crystal diamond substrate wafer is optionally located on atleast one spacer element disposed between the substrate holder and thesynthetic single crystal diamond substrate wafer to form a gas gap,wherein gas is supplied to the gas gap, and wherein a flow rate of saidgas is controlled to be no more than 5%, 4%, 3%, 2%, or 1% of a flowrate of the process gas fed into the reactor. This allows for improvedtemperature control. As a further option, the gas supplied to the gasgap is composed of gas types which are also fed into the reactor asprocess gases.

The reactor is optionally operated at a power density in the range 150to 600 Wcm⁻² of the surface of the substrate.

The synthetic diamond material optionally has an area ranging from 16mm² to 18000 mm².

The reactor substrate holder may require re-processing between syntheticdiamond growth runs in order to maintain a profile of a supportingsurface of the substrate holder.

The height of the substrate holder within the reactor is optionallyadjusted between synthetic diamond growth runs to account for materialremoved from the substrate holder by re-processing and maintain asubstantially constant height of the surface of the substrate within thereactor during subsequent synthetic diamond growth runs utilizing thesame substrate holder. The height of the surface is maintained withineither 2 mm, 1 mm, 0.8 mm, 0.5 mm, 0.3 mm, or 0.2 mm of a target heightof the surface of the substrate within the reactor.

As an option, the reactor is inverted whereby a base of the reactorsupporting the substrate forms an upper wall of the reactor relative toearth.

As an option, the growth temperature is selected from any of at least925° C., at least 950° C., at least 1000° C. and at least 1050° C.

According to a second aspect, there is provided a CVD single crystaldiamond comprising an intrinsic dislocation density of at least 10⁷cm⁻², a thickness of at least 0.5 mm, a largest linear dimension of atleast 4 mm, and a silicon concentration of no more than 5×10¹⁷atoms/cm³.

As an option, the silicon concentration is selected from no more than10¹⁷ atoms/cm³, no more than 5×10¹⁶ atoms/cm³, and no more than 10¹⁷atoms/cm³.

As an option, the CVD single crystal diamond is heteroepitaxially grown.

As an option, the CVD single crystal diamond has:

-   -   a low optical birefringence, indicative of low strain, such that        when a sample of the material is prepared as an optical plate        having a thickness of at least 0.5 mm thickness and measured at        room temperature, nominally 20° C., over an area having a        largest linear dimension of at least 25 mm, |sin δ|, the modulus        of the sine of the phase shift, for at least 98% of the measured        area of the sample remains in first order, such that δ does not        exceeding π/2, and |sin δ| does not exceed 0.9; and    -   a low and uniform optical absorption such that a sample of a        specified thickness of at least 0.5 mm has an optical absorption        coefficient at a wavelength of 1.06 μm of less than 0.09 cm⁻¹.

The CVD single crystal diamond material optionally has a largest lineardimension of is at least 25 mm or at least 100 mm.

As an option, |sin δ| does not exceed 0.9 over 100% of the measured areaof the sample.

The CVD single crystal diamond material optionally comprises a singlesubstitutional nitrogen concentration as measured by electronparamagnetic resonance (EPR) of at least 1×10¹³ atoms cm⁻³ and no morethan 5×10¹⁸ cm⁻³. As a further option, the single substitutionalnitrogen concentration as measured by electron paramagnetic resonance(EPR) is at least 3×10¹⁵ atoms cm⁻³ and no more than 5×10¹⁷ cm⁻³.

As an option, the CVD single crystal diamond material displays a 737 nmphotoluminescence to Raman peak area ratio when excited using a 660 nmlaser excitation source at 77 K selected from any of less than 10, lessthan 5, less than 1 and less than 0.1. This indicates a very lowpresence of silicon.

As an option, a largest growth surface of the material is orientedsubstantially on a {100} plane.

BRIEF DESCRIPTION OF DRAWINGS

Non-limiting embodiments will now be described by way of example andwith reference to the accompanying drawings in which:

FIG. 1 is a flow diagram showing exemplary steps to produce CVD singlecrystal synthetic diamond material;

FIG. 2 is a flow diagram showing exemplary steps to produce a syntheticdiamond wafer on which to grow further synthetic diamond material;

FIG. 3 is a micrograph of a side elevation cross section of exemplarydiamond material; and

FIG. 4 is a micrograph of a side elevation cross section of furtherexemplary diamond material.

DETAILED DESCRIPTION

Large area substrates are required in order to grow large area singlecrystal CVD synthetic diamond. As discussed above, large area singlecrystal diamond can be grown using techniques such as heteroepitaxialgrowth. Heteroepitaxially grown diamond typically has a high density ofdislocations. For example, type Ib single crystals typically havebetween 10⁴ and 10⁶ dislocation per cm². In contrast, figures of greaterthan 10⁷ dislocations per cm² have been reported for heteroepitaxiallygrown diamond in Shreck et. al., Appl. Phys. Lett. 78, 192 (2001).However, even diamond with such a high dislocation density can be usedfor some applications including heat spreading, optical applications,machining applications and synthetic gemstones. However, an even greaterrange of applications can be addressed if the heteroepitaxially grownCVD diamond is separated from its non-diamond substrate and itself usedas a substrate for further CVD growth.

FIG. 1 herein is a flow diagram showing exemplary steps to produce CVDsingle crystal synthetic diamond material. The following numberingcorresponds to that of FIG. 1.

S1. A freestanding synthetic single crystal diamond substrate wafer isprovided. The substrate wafer has a dislocation density of at least 10⁷cm⁻² as measured by counting etch pits in a given surface area afterapplying an oxygen plasma etch. An example of such a wafer is one thatis prepared by heteroepitaxial growth. In some cases, it may beadvantageous to process a surface of the substrate wafer to reducesurface damage. Examples of processing include polishing, chemicalmechanical polishing, etching, and laser processing. Processing may alsoinclude removing a non-diamond material from the single crystal diamondsubstrate wafer.

S2. The substrate wafer is located over a substrate holder within a CVDreactor.

S3. Process gases are fed into the reactor. Such process gases typicallyinclude methane and hydrogen and a plasma is formed.

S4. Synthetic diamond is grown at a temperature of least 900° C. on asurface of the substrate wafer to a thickness of at least 0.5 mm. Thegrowth process may use a power density sufficient to achieve a growthrate of at least 4 μm per hour.

After the synthetic diamond has been grown, the reactor substrate holdermay be processed to ensure that its surface is clean and to maintain aprofile of a supporting surface of the substrate holder. A height of thesubstrate holder within the reactor may be adjusted to ensurereproducibility between synthetic diamond growth runs.

FIG. 2 is a flow diagram showing steps of an exemplary method to obtaina synthetic diamond substrate with a dislocation density of at least 10⁷cm⁻². The following numbering corresponds to that of FIG. 2.

S5. A non-diamond substrate is located over a substrate holder within areactor, the non-diamond substrate comprising a surface on which thesynthetic single crystal diamond substrate wafer is to be grownheteroepitaxially. Examples of a non-diamond substrate include any ofiridium, silicon and silicon carbide.

S6. Process gases (typically methane and hydrogen) are fed into thereactor and a plasma is formed.

S7. The synthetic single crystal diamond substrate wafer is then grownon a surface of the non-diamond substrate.

It should be noted that while the above description refers to growingsingle crystal synthetic diamond on a heteroepitaxially grown singlecrystal synthetic diamond wafer substrate, other types of single crystaldiamond wafer substrates may be used.

Growth on large area single crystal synthetic diamond wafer substratescan be problematic owing to stresses building up and cracking thesynthetic diamond. This can be alleviated to a certain extent by any ofthe following techniques:

1. Subsurface damage can be removed from the single crystal syntheticdiamond wafer substrates before growth. This can be done, for example,by scaife polishing.

2. Subsurface damage can be removed from the single crystal syntheticdiamond wafer substrates before growth by plasma etching, such asinductively coupled plasma etching, as described in Diamond and RelatedMaterials, 18, 808-815 (2009).

CVD synthesis conditions are typically controlled such that thefreestanding single crystal synthetic diamond wafer substrate is held ata temperature of at least 900° C. If the temperature of the growthsubstrate is too low then growth rates are low. An upper limit to thegrowth temperature of 1200° C. is generally required to avoiddetrimental defect formation in the CVD layer, such as twins.Furthermore, the substrate temperature, in combination with otherparameters such as carbon containing gas concentration, affects themorphology of the growing single crystal CVD synthetic diamond materialand thus can be selected and controlled to achieve a desired morphology.

The inventors have discovered that, for CVD growth on substrates with ahigh dislocation density, such as those observed in heteroepitaxialdiamond, to avoid cracking of the growing CVD diamond, a temperatureabove 900° C., is required. Without being bound by any specific theory,it is suggested that such conditions lead to a reduction of thedifference in strain between the high dislocation density substrate andthe overgrown CVD layer.

CVD synthesis conditions are controlled such that a CVD synthesisatmosphere comprises a carbon containing gas (e.g. methane) at aconcentration by volume in a range 3 to 8%, more preferably in a range 4to 6%. If the carbon containing gas concentration is too low then growthrates are too low. If the carbon containing gas concentration is toohigh then cracking may occur and/or the material may have a poor opticalquality. Furthermore, as previously stated, carbon containing gasconcentration, in combination with other parameters such as thesubstrate temperature, affects the morphology of the growing singlecrystal CVD synthetic diamond material and thus is selected andcontrolled to achieve the desired morphology close to net shape of thefinal processed product.

CVD synthesis conditions are further controlled to provide a high powerdensity across the substrate of at least 150 W/cm², 180 W/cm², 200W/cm², 230 W/cm², 250 W/cm², 270 W/cm², 290 W/cm², 310 W/cm², or 330W/cm². The power density will generally be less than 600 W/cm², 500W/cm², or 400 W/cm². In the context of this specification, power densityis defined as the total microwave input power divided by the area of thesubstrate, or the substrate holder, whichever has the greater area.

The CVD synthesis conditions are controlled such that the single crystalCVD synthetic diamond material is grown at a growth rate of at least 5μm/hr, 6 μm/hr, 7 μm/hr, 8 μm/hr, 9 μm/hr, 10 μm/hr, 11 μm/hr, 13 μm/hr,16 μm/hr, or 19 μm/hr. The growth rate will generally be less than 40μm/hr, 30 μm/hr, or 25 μm/hr. While high growth rates are desired foreconomic reasons, if the material grows too quickly it can be of pooroptical quality and may be prone to cracking.

In addition to the above, high, axially oriented, process gas flow ratescan be used to further increase growth rates. For example, CVD synthesisconditions can be controlled such that a total process gas flow rate isat least 0.5, 1, 3, 5, 10, 15, 20, or 25 standard litres per minute asdescribed, for example, in WO2012/084656. Generally, total process gasflow rate will not usually exceed 100 standard litres per minute.

CVD synthesis conditions are controlled such that the single crystal CVDsynthetic diamond material is grown to a thickness of at least 0.5 mm,0.8 mm, 1.0 mm, 1.3 mm, 1.5 mm, 2 mm, or 2.5 mm and that a processedproduct can be fabricated which has a thickness of at least 0.5 mm, 0.8mm, 1.0 mm, 1.3 mm, 1.5 mm, 2 mm, or 2.5 mm. The upper limit for thethickness of the single crystal CVD synthetic diamond product willdepend on its end application but will generally be no more than 10 mm.The as-grown single crystal CVD synthetic diamond material can then beprocessed. Processing includes surface processing to convert theas-grown material into a product. Examples of processing techniquesinclude one or more of cutting, cleaving, lapping, polishing, and/oretching. Each processed single crystal CVD synthetic diamond may beformed from at least 50%, 60%, 70%, 80%, or 90% by volume of itsassociated as-grown single crystal CVD synthetic diamond.

The following examples illustrate ways in which single crystal syntheticdiamond material can be produced:

Example 1

Heteroepitaxially grown single crystal synthetic diamond was obtainedhaving and a dislocation density on a surface over 10⁷ cm⁻². Thismaterial was used as a freestanding synthetic single crystal diamondsubstrate wafer, and did not include any non-diamond material such as asilicon substrate. The dislocation density was measured by applying anoxygen-containing plasma to the surface of the diamond. This forms etchpits that reveal the presence of dislocations. The number of etch pitsin a predetermined area are counted to determine the dislocationdensity.

The freestanding synthetic single crystal diamond substrate wafer wasplaced in a microwave CVD reactor and diamond was grown on a surface ofthe freestanding synthetic single crystal diamond substrate wafer usinga feed gas of methane and hydrogen, a power in a range of 3 to 60 kW, apressure in a range of 90 to 400 torr and a growth temperature in arange of 900° C. to 1050° C. The feed gas contained 0.6 ppm N₂. Theresultant single crystal synthetic diamond was grown to a thickness ofgreater than 0.5 mm.

The single substitutional nitrogen content [N_(s) ⁰] of the grown singlecrystal diamond was measured using UV-Visible absorption spectroscopy(described in WO 03/052177), and found to be 150 ppb. The singlesubstitutional nitrogen content [N_(s) ⁰] can also be measured byelectron paramagnetic resonance (EPR).

Birefringence was measured using a technique similar to that describedin WO2004/046427 using a pixel size area in a range of 1×1 μm² to 20×20μm². A maximum |sin δ| was found to be around 0.8 for light having awavelength of 550 nm using a sample of 3×3×0.5 mm. The Δn_([average]),the average value of the difference between the refractive index forlight polarised parallel to the slow and fast axes averaged over thesample thickness, was found to be 1.6×10⁻⁴.

Example 2

Heteroepitaxially grown single crystal synthetic diamond was obtainedhaving a dislocation density on a growth surface of greater than 10⁷cm⁻². This material was used as a freestanding synthetic single crystaldiamond substrate wafer, and did not include any non-diamond materialsuch as a silicon substrate.

The freestanding synthetic single crystal diamond substrate wafer wasplaced in a microwave CVD reactor and diamond was grown on a surface ofthe freestanding synthetic single crystal diamond substrate wafer usinga feed gas of methane and hydrogen, and a power, pressure andtemperature in the same ranges as described in Example 1. The feed gascontained 2.9 ppm N₂. The resultant single crystal synthetic diamond wasgrown to a thickness of greater than 0.5 mm.

The single substitutional nitrogen content [N_(s) ⁰] of the grown singlecrystal diamond was found to be 150 ppb. Birefringence was measuredusing a pixel size area in a range of 1×1 μm² to 20×20 μm². A maximum|sin δ| was found to be around 0.8 for light having a wavelength of 550nm using a sample of 3×3×0.5 mm. The Δn_([average]), the average valueof the difference between the refractive index for light polarisedparallel to the slow and fast axes averaged over the sample thickness,was found to be 1.6×10⁻⁴.

Example 3

A set of five heteroepitaxially grown CVD diamond substrates were usedfor subsequent CVD diamond synthesis. The substrates were of size4.0×4.0×0.3 mm³ and of nominally the same defect concentrations. Allfaces of the substrates were {100} and were scaife polished in order toproduce a low damage surface finish.

A selection of representative substrates were subjected to anoxygen-containing plasma etch in order to form etch pits and thus revealthe presence of the dislocations. Using this method, the dislocationdensity at the growth face of the substrate was estimated to be greaterthan 10³ cm⁻².

These substrates were included in a series of CVD diamond synthesis runsin a 2.45 GHz microwave plasma CVD diamond reactor and using a mixtureof hydrogen, argon, methane and nitrogen feed gases, in which the onlyprocess parameter varied run-to-run was the substrate temperature. Theprocess conditions used (other than substrate temperature were: powerdensity=224 W cm⁻²; pressure 230 Torr; hydrogen flow=600 sccm; methaneflow=40 sccm; argon flow=20 sccm; nitrogen gas phase concentration=3ppm. A number of heteroepitaxial substrates were included in each run.

After CVD diamond synthesis, all stones were imaged using a low poweroptical microscope and the number of major cracks visible were counted.In addition, the CVD growth thickness was measured. From these values,an average number of cracks and an average thickness was calculated.These values are shown in Table 1. For synthesis run 3.1 (having lowestgrowth temperature) the crack density was such that only an estimate waspossible.

TABLE 1 Example 3 Substrate Average CVD Average number temperaturegrowth thickness of macroscopic Example (° C.) (mm) cracks 3.1 8001.4 >20 3.2 870 1.7 6.7 3.3 890 1.7 3.0 3.4 925 1.8 0 3.5 950 1.6 0

Growth rates of between 4 μm per hour and 16 μm per hour were observed.These growth rates may be improved by altering the temperature, pressureand gas chemistry.

It can be seen that having a growth temperature of at least 900° C.eliminated the number of cracks that could be measured.

Diamond temperature during growth was measured using a one-colourpyrometer operating at 2.2 μm and assuming a diamond emissivity of 0.9.Diamond emissivity changes with temperature, but the method givesrepeatable results. The diamond wafer substrate is brazed to a carrier.During growth, polycrystalline diamond forms on the carrier. Thepyrometer is aimed at the polycrystalline material next to the singlecrystal diamond.

FIG. 3 shows a side elevation cross section of Example 3.2, grown at870° C. The sample consists of the single crystal diamond substratewafer 1 and the grown synthetic diamond material 2. Several cracks canbe seen in the synthetic diamond material 2, including a very largecrack 3.

FIG. 4 shows a side elevation cross section of Example 3.5, grown at950° C. The sample consists of the single crystal diamond substratewafer 4 and the grown synthetic diamond material 5. It can be seen thatthe synthetic diamond material 5 material is crack free.

The single substitutional nitrogen content of the samples was measuredby UV-Vis and found to vary between 140 and 160 ppb. The siliconconcentration for all of the samples was no more than 5×10¹⁷ atoms/cm³.The presence of silicon is further indicated by a photoluminescence peakin diamond at 737 nm using a 660 nm laser excitation source at 77 K. Theratio of area of this peak to the area of the Raman peak was found to beno more than 10. In some samples it was found to be no more than 0.1

Considering example 3.4, where no cracks were observed, an alphaparameter of 1.32 was measured. The alpha parameter is defined as

$\alpha = {\sqrt{3}\frac{{GR}_{100}}{{GR}_{111}}}$

Where GR₁₀₀ is the linear growth rate in the <100> direction and GR₁₁₁is the linear growth rate in the <111> direction. This is described inmore detail in Silva et. al., Journal of Crystal Growth 310 (2008)187-2003. The growth parameter can be used to predict and optimisefactors such as the growth rate, largest usable diamond surface area andso on, and be used to control diamond crystal morphology during growth,which in turn can be used to minimise stresses within a single crystaldiamond. {111} growth faces can appear at the corners of the growingdiamond material leading to certain structural defects, and so it isdesirable that the {100} planes grow in preference to the {111} planes.

A number of potential applications are envisaged for the product asdescribed herein. For example, the single crystal CVD synthetic diamondmaterial may be used to form an optical prism or a mechanical toolcomponent with the tip forming a cutting edge or point.

While this invention has been particularly shown and described withreference to embodiments, it will be understood by those skilled in theart that various changes in form and detail may be made withoutdeparting from the scope of the invention as defined by the appendedclaims. For example, while the examples describe a heteroepitaxiallygrown diamond substrate, other single crystal diamond materialsubstrates could be used. Furthermore, where the above descriptionrefers to the diamond being grown on a carrier with a spacer to form agas gap, it is thought to be possible to do away with a carrieraltogether and just provide the diamond substrate on spacers to form agas gap.

1. A method of manufacturing synthetic single crystal diamond materialusing a chemical vapour deposition process, the method comprising:providing a freestanding synthetic single crystal diamond substratewafer having a dislocation density of at least 10⁷ cm⁻²; locating thesynthetic single crystal diamond substrate water over a substrate holderwithin a chemical vapour deposition reactor; feeding process gases intothe reactor, the process gases including a gas comprising carbon;growing synthetic diamond material on a surface of the single crystaldiamond substrate wafer at a growth temperature of at least 900° C. to athickness of at least 0.5 mm and with lateral dimensions of at least 4mm by mm such that the grown synthetic diamond material is crack free.2. (canceled)
 3. The method according to claim 1, wherein the processgases comprise hydrogen and not more than 5% oxygen.
 4. (canceled) 5.(canceled)
 6. The method according to claim 1, further comprisingproviding the synthetic single crystal diamond substrate wafer by thesteps of: locating a non-diamond substrate over a substrate holderwithin a further reactor, the substrate comprising a surface on whichthe synthetic single crystal diamond substrate wafer is to be grownheteroepitaxially; feeding process gases into the further reactor;growing the synthetic single crystal diamond substrate wafer on thesurface of the non-diamond substrate.
 7. (canceled)
 8. The methodaccording to claim 6, wherein the non-diamond substrate comprises any ofiridium, silicon and silicon carbide.
 9. The method according to claim3, further comprising removing a non-diamond substrate from thesynthetic single crystal diamond substrate wafer prior to locating thesynthetic single crystal diamond substrate wafer over the substrateholder.
 10. (canceled)
 11. (canceled)
 12. The method according to claim1, wherein the grown synthetic diamond material is oriented atsubstantially {100} relative to the synthetic single crystal diamondsubstrate wafer.
 13. (canceled)
 14. (canceled)
 15. The method accordingto claim 1, wherein the reactor is operated at a power density in therange 150 to 600 Wcm⁻² of the surface of the substrate.
 16. The methodaccording to claim 1, comprising growing the synthetic diamond materialhaving an area from 16 mm⁻² to 18000 mm⁻².
 17. (canceled)
 18. (canceled)19. (canceled)
 20. The method according to claim 1, wherein the growthtemperature is selected from any of at least 925° C., at least 950° C.,at least 1000° C. and at least 1050° C.
 21. A CVD single crystal diamondcomprising: an intrinsic dislocation density of at least 10⁷ cm⁻²; athickness of at least 0.5 mm; a largest linear dimension of at least 4mm; and a silicon concentration of no more than 5×10¹⁷ atoms/cm³. 22.The CVD single crystal diamond according to claim 21, wherein thesilicon concentration is selected from no more than 10¹⁷ atoms/cm³, nomore than 5×10¹⁶ atoms/cm³, and no more than 10¹⁷ atoms/cm³. 23.(canceled)
 24. The CVD single crystal diamond according to claim 21,comprising: a low optical birefringence, indicative of low strain, suchthat when a sample of the material is prepared as an optical platehaving a thickness of at least 0.5 mm thickness and measured at roomtemperature, nominally 20° C., over an area having a largest lineardimension of at least 25 mm, |sin δ|, the modulus of the sine of thephase shift, for at least 98% of the measured area of the sample remainsin first order, such that d does not exceeding π/2, and |sin δ| does notexceed 0.9; and a low and uniform optical absorption such that a sampleof a specified thickness of at least 0.5 mm has an optical absorptioncoefficient at a wavelength of 1.06 pm of less than 0.09 cm⁻¹.
 25. TheCVD single crystal diamond material according to any one of claims 21 to24, wherein the largest linear dimension is selected from at least 25 mmor at least 100 mm.
 26. (canceled)
 27. (canceled)
 28. (canceled)
 29. TheCVD single crystal diamond material according to claim 21, comprising a737 nm photoluminescence to Raman peak area ratio when excited using a660 nm laser excitation source at 77 K selected from any of less than10, less than 5, less than 1 and less than 0.1.
 30. The CND singlecrystal diamond material according to claim 21, wherein a largest growthsurface of the material is oriented substantially on a {100} plane.